1. Field of the Invention
The present invention relates to a magnetoresistance effect element and a magnetic random access memory.
2. Related Art
Various types of solid magnetic memories have been developed. In recent years, magnetic random access memories (MRAM) including magnetoresistance effect elements each exhibiting the giant magnetoresistive (GMR) effect have been suggested, and, particularly, attention is drawn to magnetic random access memories using ferromagnetic tunnel junctions each exhibiting the tunnel magnetoresistive (TMR) effect.
A MTJ (Magnetic Tunnel Junction) element of a ferromagnetic tunnel junction is formed with a three-layer film including a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer. At the time of reading, a current flows, tunneling through the insulating layer. The junction resistance value at this point varies depending on the cosine of the relative angle between the magnetization of the first ferromagnetic layer and the magnetization of the second ferromagnetic layer. Accordingly, the junction resistance value becomes smallest when the magnetization directions of the first and second ferromagnetic layers are parallel to each other (the same direction), but becomes largest when the magnetization directions of the first and second ferromagnetic layers are antiparallel to each other (opposite from each other). This is called the TMR effect. The variation in resistance value due to the TMR effect sometimes becomes greater than 300% at room temperature.
In a magnetic memory device that includes MTJ elements of ferromagnetic tunnel junctions as memory cells, at least one of the ferromagnetic layers in each memory cell is regarded as the reference layer, and the magnetization direction of the ferromagnetic layer is fixed, while the other ferromagnetic layer is set as the recording layer. In such a cell, information is recorded by associating binary information of “0” and “1” with the parallel magnetization arrangement and antiparallel magnetization arrangement between the reference layer and the recording layer. Conventionally, writing of recording information is performed on such cells according to a technique by which the magnetization of the recording layer is reversed with a magnetic field generated by applying a current to the write wire provided separately from the cell (the technique will be hereinafter referred to as the current field write technique). By the current field write technique, however, the current amount required for writing becomes greater as the memory cells become smaller. As a result, it becomes difficult to achieve large memory capacity. In recent years, a technique for replacing the current field write technique has been suggested (see U.S. Pat. No. 6,256,223, for example). By the technique, the magnetization of the recording layer is reversed with spin torque injected from the reference layer by application of a current directly to each MTJ element (the technique will be hereinafter referred to as the spin torque write technique). By the spin torque write technique, the current amount required for writing characteristically becomes smaller as the memory cells become smaller in size, and large memory capacity can be easily achieved. Reading information from a memory cell is performed by applying a current to the ferromagnetic tunnel junction and detecting the resistance variation caused by the TMR effect. Such memory cells are arranged in large number, so as to form a magnetic memory. An actual structure is formed by arranging switching transistors for the respective cells, as in a DRAM, for example, so that a desired cell can be selected, and then incorporating a peripheral circuit into the structure.
To realize large-capacity memory, it is necessary to increase the cell occupancy in the chip by making the MTJ elements smaller in size, and to reduce the current amount necessary for writing. For example, to realize large-capacity memory of several gigabits or larger, the write current density should be smaller than 1 MA/cm2. As mentioned above, the spin torque write technique is advantageous in realizing large-capacity memory. However, the current density required for writing is reportedly in the neighborhood of 3 MA/cm2, and the decrease in write current amount is not sufficient.